The most widely held concept of active transport by cells is that their cell membranes possess a descrete number of energy coupled transport molecules, or sites, which mediate vectorial fluxes of specific substrates. The main thrust of this proposal is the localization of these transport sites at the cellular and membrane levels. Specific goals are measurement of renal tubular and fish gill distributions of Na ion, K ion ATPase, the molecule which is thought to be the primary mediator of salt transport in these tissues. H3-ouabain autoradiography will be used to localize this enzyme. Carbonic anhydrase is another enzyme thought to play a key role in electrolyte transport. Its distribution in renal tubles will be measured using autoradiography of tritium labelled inhibitors, acetozolimide. In another area, efforts are being made to determine the mechanism by which mercurials and other-SH reagents (e.g. NEM) inhibit sugar and amino acid transport in the small intestine. Specifically, we wish to determine whether or not organomercurials, Hg ions and NEM react with common membrane sites. This will be assessed by measuring ingibition of unidirectional nucosal fluxes of these substrates in rabbit ileum. BIBLIOGRAPHIC REFERENCES: Stirling, Charles E. mercurial Perturbation of Bruch Border Membrane Permeability in Rabbit Ileum, J. Membrane Biol. 23, 33-56 (1975). Stirling, Charles E. High Resolution Autoradiography of H3-Ouabain Binding in Salt Transporting Epithelia, Journal Microscopy 106, Pt 1 January 1976, (in press).